Sauropodomorpha

Introduction

Prosauropods and sauropods share a close ancestry and are characterized by their small heads and long necks. Together they make up the Sauropodomorpha clade of the lizard-hipped Saurischia order, which is, in turn, one of the two orders of Dinosauria.

Although sauropodomorphs have been found on every continent and their history spans from the beginning of dinosaur existence in the Late Triassic, about 230 million years ago, to the end of all non-avian dinosaurs in the Late Cretaceous, about 66 million years ago, their fossil record remains relatively poor. Much that is known comes from very few basal (earliest) forms or incomplete skeletons. Some paleontologists believe that the relatively poor showing of sauropodomorph fossils is because their sheer size made burial difficult, and without burial of some kind, fossils cannot be created.

Fast Facts

Pronunciation: Sauropodomorpha (pronounced sawr-oh-POH-dah-more-fah), meaning “lizard feet forms”

Time Period: Late Triassic (235–201 million years ago) to Late Cretaceous (100–66 million years ago)

Size: 5–60 m (16–196 ft) in length

Weight: 10 to 120 metric tons (22,000–265,000 lbs)

Diet: Herbivorous

Location: All continents

Lifespan: Up to 50 years, although some paleontologists believe up to 100 years or more

Homologous Traits

Sauropodomorphs possess the characteristics of the archosaurs—four limbs, oviparous reproduction, backbones, and upper and lower temporal skull openings—as well as the traits common to all dinosaur species, specifically upright posture, modified fourth and fifth digits on the hands, three-toed feet, specialized crests on the humerus and tibia bones, three or more sacral vertebrae, and a femur with a ball-like head at one end.

Sauropodomorphs are specifically characterized by their small heads and mouths and their long necks and tails. Other physical traits shared among these creatures include one to three extra sacral vertebrae, very large “thumbs” (pollex) with equally large claws, and simple crowned teeth that are leaf-shaped in prosauropods, spatulate in basal sauropods, and pencil-shaped in later sauropods such as camarasaurs and titanosaurs.

A few Sauropodomorpha fossils have also been found with gastroliths. Swallowing these “stomach stones” to aid digestion is thought to be a typical behavior of sauropodomorphs and can be seen today in several species of birds. Fossilized stomach stones found in dinosaur species are often very smooth, and it is believed that the stones stayed in the stomachs of these dinosaurs for many years.

Sauropodomorphs are typically considered to be at the lower end of the dinosaur intelligence scale, as their brain-to-body size ratio (known as the encephalization quotient) is particularly small. This is not surprising; even in modern-day animals, carnivores tend to be more intelligent than true herbivores. It is believed that predators require higher intelligence and the associated behavioral flexibility and learning capacity in order to hunt successfully. Theropod dinosaur predators were also generally faster than sauropodomorphs would have been, with their slow, plodding gait.

Evolutionary Divergences

Sauropodomorphs are classified as herbivorous species, typified by their long necks and tails and small heads and brains. Although phylogenetic debate continues, sauropods are thought to have evolved from basal sauropodomorphs, with the early sauropods not unlike the earlier, smaller prosauropods such as Anchisaurus and Massospondylus.

Prosauropods lived from the Late Triassic (235–201 million years ago) to the Early Jurassic (201–174 million years ago) and have been found on every continent. Prosauropods and sauropods share a close ancestry and together make up the Sauropodomorpha clade of the order Saurischia, one of the two orders of Dinosauria. Generally, the differences between prosauropods and sauropods relate to their size, neck length, arm length, locomotion, and tooth shape. It is believed, for example, that sauropods were obligate quadrupeds, while prosauropods may have been obligate quadrupeds or facultative bipeds with longer forelimbs, suggesting adaptation to different ecological habits.

Paleontologists believe that all sauropodomorphs, from the basal prosauropods to the more advanced sauropods, were obligate herbivores. This means that they required plant-based foods to survive and thrive and likely had to eat almost constantly in order to obtain enough energy from their relatively low-nutrition diet. The commonly seen pencil-like teeth of sauropods and leaf-shaped teeth of prosauropods are well suited to raking and shearing vegetation, rather than the chewing action of carnivores. Researchers believe that sauropodomorph digestion was aided not by intense chewing and grinding, but by a huge alimentary canal and the presence in some sauropodomorphs of gastroliths and large amounts of bacteria in the stomach.

Sauropodomorpha evolved vertebrae with cavities (pleurocoels) to help lighten the bones in their long necks. The extremely long necks of sauropodomorphs, more specifically the sauropods—as evidenced in species such as Mamenchisaurus, whose neck reached an impressive 14 meters (46 ft) in length—are believed to be an evolutionary feeding adaption and possibly coevolved with the increased height of forest canopies during the Mesozoic. However, recent evidence casts some doubt on this, as up-and-down neck movement may have been more limited than previously thought, suggesting that these species foraged closer to the ground in wide, sweeping horizontal movements.

Prosauropods went extinct during the Jurassic. Sauropods were much more successful in terms of longevity, finally going extinct at the end of the age of the dinosaurs in the Late Cretaceous, some 66 million years ago. Titanosaurids were the last of the sauropods, thriving in Africa, Asia, and South America until, like all sauropodomorphs, they died out in the mass extinction event that marked the Late Cretaceous–Paleogene boundary.

Creatures in This Group

Paleontologists generally favor cladistic classification, as it provides a better understanding of evolutionary relationships in fossil-poor species. All dinosaurs are divided into either Ornithischia (“bird-hipped” dinosaurs) or Saurischia (“lizard-hipped” dinosaurs). Sauropodomorphs belong to the Saurischia order and are the sister clade of the predominantly carnivorous bipedal theropods.

Prosauropods are basal sauropodomorphs, likely sharing a common ancestor with the sauropods, but divided from them by their somewhat smaller stature and longer forelimbs. Prosauropods are the earliest known sauropodomorphs and were the dominant herbivores of the Late Triassic and Early Jurassic. Species include the African and North American Massospondylus, a midsized herbivore with a small head and a long neck that lived during the Early Jurassic; Thecodontosaurus, a relatively small herbivore from the Late Triassic; and the 6 to 8 meter (20–26 ft) long Plateosaurus of the Late Triassic, a member of the Anchisauridae family, although current opinion suggests that this species may be more closely related to sauropods than to prosauropods.

Sauropods comprise several nodes: Eusauropoda, Neosauropoda, Camarasauromorpha, Titanosauriformes, Lithostrotia, and Saltasauridae. The Neosauropoda clade, which consists of Diplodocoidea (Diplodocidae, Dicraeosauridae, and Rebbachisauridae) and Macronaria (Camarasauridae, Brachiosauridae, and Titanosauria), was derived from the basal sauropods. The grouping of fossil finds into clades is most often based on physical traits such as skull shape and size, length of neck, gait, dentition, and armor. However, new discoveries and technologies influence the renaming, reclassification, and sometimes validity of species, nodes, clades, and occasionally whole families.

The Eusauropoda clade is made up of large sauropods with small heads, rounded snouts, and long necks. Members of Nesosauropoda, a division of Eusauropoda, possess higher nostrils and teeth grouped at the front of their jaw. Within Nesosauropoda are Diplodocidae (including diplodocids such as the North American Apatosaurus, Barosaurus, and Diplodocus from the Late Jurassic, and the Argentinean Amargasaurus, Mongolian Nemegtosaurus, and Moroccan Rebbachisaurus from the Cretaceous) and Macronaria (including the Moroccan Atlasaurus and the Chinese Bellusaurus from the Middle Jurassic).

Camarasauromorpha is an unranked clade, generally considered to be the common ancestor of Camarasaurus and Saltasaurus and often regarded as synonymous with Macronaria. This clade contains the families Camarasauridae (including Camarasaurus in the western United States, from the Late Jurassic) and Saltasauridae (including the Argentinian Saltasaurus from the Late Cretaceous).

The sauropods within the Titanosauriformes clade are some of the heaviest, which is not surprising, as this includes the famous Brachiosauridae family (to which belongs the massive Brachiosaurus from the Late Jurassic, found in Africa, the western United States, and Portugal) and the Titanosauridae family (including Titanosaurus from the Late Cretaceous in India). Titanosaurids were, in fact, the last of the sauropods, thriving in Africa, Asia, and South America until the mass extinction event. The Lithostrotia clade is also thought to belong to the Titanosauriformes and include those sauropods that possessed dermal armor.

Ecology

Perhaps the largest misapprehension regarding sauropodomorphs is the belief that they lived submerged in water habitats, as suggested by their large size, long necks, and nostril position. Research has shown that such a lifestyle was impossible, as the pressure in water deep enough to submerge these animals would have also prevented them from breathing. In addition, their strong pillar-like legs, similar to those of modern-day elephants, would have been well suited to walking across the terrestrial landscape, rather than swimming in deep lakes.

Fossilized sauropodomorph teeth show signs of scratching, which researchers believe was caused by grit and dirt abrasion from eating plants with soil attached. Such abrasions suggest that some species of sauropodomorphs may not have fed on the high treetop vegetation ideal for the long-necked sauropods, preferring plants situated close to the ground instead. By delineating low-feeding grazers from high-feeding browsers, such fossil markings help determine whether these species exhibited niche overlap or niche segregation, and thus how such large species coexisted in a resource-limited environment.

Dinosaur trackways (fossilized footprints) found in American, Tanzania, India, and China provide strong evidence that many sauropodomorphs lived in herds, unlike the generally carnivorous and solitary theropods. Paleontologists believe that some sauropodomorph herds were segregated by age, with adults and juveniles forming separate groups, possibly to reduce food competition by resource partitioning. Sauropodomorph herding may have also been associated with migratory behavior. Such large animals would have caused significant damage to the local vegetation by trampling and foraging, and probably migrated to new, undamaged areas in order to feed. It is likely that these species were instrumental in shaping the ancient terrestrial landscape.

The oldest dinosaur eggs date to the Late Triassic and are believed to belong to the prosauropod Coloradisaurus. All dinosaurs, including sauropodomorphs, were oviparous and laid eggs in vegetation along the ground, or perhaps buried them. Although it is considered unlikely that sauropodomorphs exhibited prehatching brooding behavior, adults may have protected breeding sites and nests. Given their herding behavior, sauropodomorphs would have displayed posthatching care and protection of young and juveniles.

Paleontology News

Sauropodomorpha (meaning “lizard feet forms”) was first established by German paleontologist Friedrich von Huene in 1932, who further divided it into the basal Prosauropoda and the descendant Sauropoda. Despite this early division, the most often debated aspect of Sauropodomorpha classification is also the most fundamental. Researchers still dispute whether prosauropods should be classified as Sauropodomorpha at all, and if so, whether they are a natural monophyletic group or a direct ancestor of sauropods. The relationship between sauropods and prosauropods remains controversial, and without new fossils to shed light on these questions or provide a clearer picture of sauropodomorph evolution, it is unlikely to be resolved any time soon.

As such, although debate continues, most classifications maintain the division and consider both Sauropoda and Prosauropoda to be monophyletic, sharing a common, as-yet-unknown ancestor. The evidence for this relates to the much more reduced fifth digit seen on the hind feet of prosauropods. If sauropods descended directly from prosauropods, the “unreducing” of a reduced digit would have required an unlikely evolutionary reversal.

Bibliography

Books

Carpenter, Kenneth. Carnivorous Dinosaurs. Bloomington: Indiana University Press, 2005.

• An examination of the carnivorous theropod dinosaurs, with scientific papers on topics such as classification, anatomy, morphology, and ecology.

Curry Rogers, Kristina A., and Jeffrey A. Wilson. Sauropods: Evolution and Paleobiology. Berkeley: University of California Press, 2005.

• Focuses on sauropod evolution, anatomy, evolutionary history, herbivory, reproduction, and classification.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. New York: Cambridge University Press, 2009.

• Focuses on many aspects of the natural sciences and how they relate to dinosaur biology, evolution, life history, and classification.

—. Evolution and Extinction of the Dinosaurs. New York: Cambridge University Press, 2005.

• A discussion of complex scientific processes and dinosaur species.

Klein, Nicole, Kristian Remes, Carole T. Gee, and P. Martin Sander. Biology of the Sauropod Dinosaurs: Understanding the Life of Giants. Bloomington: Indiana University Press, 2011.

• Discusses sauropods with regard to diverse fields of research, specifically nutrition, physiology, biology, ecology, and skeletal structure and body plans.

Lucas, Spencer George. Dinosaurs: The Textbook. New York: McGraw-Hill, 2004.

• Provides a précis of important aspects of dinosaur discovery, behavior, biology, and classification.

Martin, Anthony J. Introduction to the Study of Dinosaurs. Malden, MA: Blackwell, 2006.

• Provides scientifically based chapters on the major dinosaur clades, as well as anatomical, physiological, and behavioral information.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Richmond hill, on: Firefly Books, 2009.

• Provides useful information on more than 500 dinosaurs, including discovery, location, and characteristics such as anatomy and diet.

Weishampel, David B., Peter Dodson, and Halszka Osmólska. Dinosauria. Berkeley: University of California Press, 2007.

• Presents resources and scientific papers on dinosaurs, looking especially at the Saurischia and Ornithischia orders with regard to their evolution, distribution, and ecology.

Journals

Taylor, Michael. “Sauropod Dinosaur Research: A Historical Review.” Geological Society of London 343 (2010): 361–86.

• Discusses three phases of sauropod dinosaur research, claiming that since the 1960s, the public appetite for dinosaur-related news is prompting new and more descriptive work.

—, M. J. Wedel, and D. Naish. “Head and Neck Posture in Sauropod Dinosaurs Inferred from Extant Animals.” Acta Palaeontologica Polonica 54.2 (2009): 213–20.

• Compares the neck posture of extant amniotes with that of sauropods, claiming that sauropods maximally extended their necks and maximally flexed their heads.

Additional Works Used

“Dinobase.” University of Bristol. Web. June 2011. <http://dinobase.gly.bris.ac.uk>.

Ezcurra, Martin D. “A New Early Dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: A Reassessment of Dinosaur Origin and Phylogeny.” Journal of Systematic Palaeontology 8.3 (2010): 371–425.

Martinez, R. N., and O. A. Alcober. “A Basal Sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the Early Evolution of Sauropodomorpha.”PLoS ONE 4.2 (2009): e4397.

Natural History Museum of London. Web. June 2011. <http://www.nhm.ac.uk/>.

Natural History Museum of Los Angeles. Web. June 2011. <http://dinosaurs.nhm.org/dinosaurs/>.

Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. Berkeley: University of California Press, 1995.

Pol, D., A. Garrido, and I. A. Cerda. “A New Sauropodomorph Dinosaur from the Early Jurassic of Patagonia and the Origin and Evolution of the Sauropod-type Sacrum.” PLoS ONE 6.1 (2011): e14572.

Roger, Kristina, A., and Wilson, Jeffrey, A. Sauropods: Evolution and Paleobiology. Berkeley: University of California Press, 2005.

Wilson, J. A. Sauropod Phylogeny. Web. June 2011. <http://www-personal.umich.edu/~wilsonja/JAW/Sauropod‗Phylogeny.html.>